Extracting subtitles from my own subtitled film

Hi,
I have a question about extracting subtitles (.srt) from one of my own subtitled films. I'm pretty sure the subtitles are Open Captions and not Closed Captions. I don't have the project file left since it was a couple of years ago. Is it possible to do this in any way?

Information about the video file: MP4 / 1920x810 / 25 fps.

I have tried the following tools but they don't seem to recognize the subtitle track: MediaInfo, Subrip, Mp4box.